What is a current transducer?
Current transducer basics
What would we do without electricity? Nothing! This energy is everywhere and the ongoing energy transition and decarbonization of our industry is pulling for even more electrification in all our daily activities like e-mobility for example. Like any physical quantities if you don’t measure it, you can’t manage it! Indeed in any processes the measurement is critical to monitor, to meter and to control the system and to do it a current transducer is needed.
Primary and secondary current
Basically a current transducer is a device converting the current signal we wanted to measure, called “primary” current, into another signal, called “secondary” current or voltage, usable by electronic control board or instruments. As the primary current can be different (AC or DC, few mA to kA, isolated or not…), there is a large diversity of current transducers using different technologies described below.
Flux Gate current transducers
In general, the flux gate principle is to use an excited magnetic material coil as a probe. Thanks to a saturation/desaturation cycle and signal processing, this coil is able to measure the magnetic field proportionally. From that multiple options are possible to design a current transducer. It can simply replace a Hall effect probe in the air gap or the coil could have the shape of a tore.
For this second option, the current transducer can achieve really high accuracy (few ppm) and a strong EMC robustness. For more detail we invite you to read the detailed article on the flux gate principle.
How does a current transducer work?
Measuring the electrical current directly is quite a challenging task. This is often simplified by measuring the side-effects of the passing current e.g. measuring the voltage drop created across a resistive shunt (Ohm’s law) or measuring the magnetic field surrounding the primary conductor (Ampere’s law).
Current measurement using a resistive shunt is quite straightforward, but requires that the user interrupts the primary circuit to insert the shunt, provides proper cooling of the shunt, or at least does proper shunt sizing for the required current range, with the downside of having a current measurement signal which is not isolated from potentially hazardous primary voltage levels.
Current measurement by using the generated magnetic field has the advantage of being a non-contact i.e. isolated current measurement so that the primary circuit does not need to be interrupted with the insertion of a lossy component like the resistive shunt. The magnetic field created by a passing current through a primary conductor is proportional to the applied current and therefore carries the information about the current amplitude.
The way the measured magnetic field is transformed into information about the current amplitude will depend on the measurement principle (link to “What are the different types of current transducers”), but the common advantage is that current measurement is completely isolated from the primary power circuit which often makes the important task of control and protection of complex electrical systems easier to deal with.
Dc current transducers
Not every current measurement principle based on Ampere’s law can measure DC currents. For example, the inductive current transducer and Rogowski coil are relying on the magnetic induction of current and voltage in the coil due to alternating / AC magnetic field and are effectively unable to measure DC currents.
Magnetic probes which are capable of sensing DC magnetic fields, like e.g. Hall-effect, Flux-gate, and Magneto-resistive probe can be used for building DC current transducers (often called DCCTs).
Open loop and closed loop current transducers
Both open-loop and closed-loop implementations are possible, but the application of a strong DC magnetic field in an open-loop CT can pre-magnetize the magnetic core in a way that the measurement performance for AC currents is significantly affected taking into consideration the non-linear B-H curve of the common.
Ac current transducers
Ac current transducers, also called ac current transformers and typically shortened “CTs”, are widely used as instrument transformers for current sensing in the power grid, such as generators or electrical substations. Their function is to scale down the measured current to lower levels for measuring and provide the important isolation from the high voltage power lines.
In their basic form, the ac current transformer consists of a magnetic core, often ring-shaped, wound with a wire, called the secondary winding. In the opening of the ring core the primary current passes through. The working principle of an ac current transducer is Faraday’s law of induction, stating that a change in the magnetic flux will induce a voltage in the inductor winding. This means that ac current transducers can only measure ac signals. The ac current transducer is often a passive device, used with a shunt to sense the secondary current in the output signal.
Current transducer – Working principle
The working principle of an ac current transducer is Faraday’s law of induction, stating that a change in the magnetic flux will induce a voltage in the inductor winding. This means that ac current transducers can only measure ac signals.
The ac current transducer is often a passive device, used with a shunt to sense the secondary current in the output signal.